Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
1999-04-08
2001-04-24
Lee, John D. (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling structure
C385S032000
Reexamination Certificate
active
06222964
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of tunable optical filters, especially for use in optical communications systems.
BACKGROUND OF THE INVENTION
High-speed data communications systems need to support the aggregate bandwidth requirements of current and future applications such as supercomputer interconnection, high-quality video conferencing, and multimedia traffic. It has long been clear that these bandwidth requirements can only be met by using optical transmission technologies. Many current approaches favor packet switching and ATM (asynchronous transfer mode) technology, due to their flexibility. The most promising candidate for the future hardware backbone for such networks is dense optical WDM (wavelength division multiplexing), a method of multiplexing a large number of optical data channels on a wavelength basis, i.e. each wavelength is regarded as a different channel, and is routed and manipulated separately from all other wavelengths.
Dense WDM needs advanced optoelectronic components and subsystems, capable of handling the extremely high aggregate bit rates and traffic levels demanded by modern optical data communications systems. One of the most critical components needed for implementation of WDM packet-switched systems is an ultra-fast tunable filter—a wavelength selective element in which the central wavelength of the selected bandpass can be tuned externally and dynamically at a very high rate.
Fast tunable filters are known and available commercially, but the tuning speed of all currently known types falls far short of the requirements of future and even of some current optical data transmission systems. The most common optical filters are based on classical interferometers, and include Fabry-Perot and Bragg filters. Such filters are tuned by mechanically moving the resonator structure, and the tuning speed is therefore comparatively slow—typically of the order of milliseconds, or, for the very fastest types, several tens of microseconds.
Another type of tunable filter is based on the Acousto-Optical effect. Such components depend on the interaction between an acoustic wave generated in the device, and the optical signal inputted to the component. Tunability is achieved by altering the frequency of the acoustic wave, which can be simply accomplished by altering the frequency of the electronic signal used to generate the acoustic wave. These filters are, however, polarization dependent, which causes many practical problems. Tuning speeds are reasonably high, of the order of microseconds.
Yet another tunable filter is based on a micromachined semiconductor structure, where the thickness of one of the parts of the structure is altered electrically. Here too, tuning speeds of the order of microseconds can be achieved.
The next generation packet-switched WDM networks are being designed for use with traffic throughputs of the order of Tbits/sec. Such systems therefore require switching and tuning speeds of the order of one nanosecond, and it is evident that even the fastest of the above mentioned filter technologies falls woefully short of these requirements, by about three orders of magnitude.
SUMMARY OF THE INVENTION
The present invention seeks to provide an improved high speed tunable optical filter which overcomes disadvantages and drawbacks of existing tunable optical filters, which provides tuning speeds of the order of one nanosecond, and which is capable of implementation as a low cost, high production volume monolithic component.
There is thus provided in accordance with a preferred embodiment of the invention, a tunable optical filter including at least one multiport optical coupler, to one connection port of which is inputted an optical signal, and from another one of which is outputted an optical signal to the end user, an optical transmission line of predetermined length configured as a resonator, with one of its ends connected to yet another one of the connection ports, and its other end connected to still another one of the connection ports, and with a phase modulator inserted in the above mentioned resonator such that the interaction of the phase modulated signal in the resonator with the input signal allows only signals of a preselected wavelength to be transmitted from the output port to the end user.
In accordance with another preferred embodiment of the invention, the output signal can be extracted from the resonator, by means of an additional coupler inserted into the resonator. In this case, the use of three-port couplers is sufficient.
In accordance with a further preferred embodiment of the invention, a filter with variable finesse can be provided by the use of couplers with variable power splitting ratios.
In accordance with yet another preferred embodiment of the invention, there is provided a compound resonator tunable optical filter including at least three multiport optical couplers, to one of the ports of the first coupler is inputted an optical signal, and from one of the ports of the last coupler is outputted a filtered optical signal to the end user, at least two optical resonators of predetermined length interconnecting the free ports of the couplers, and at least one phase modulator inserted in at least one of the above mentioned optical resonators, such that the interaction of the phase modulated signal with the input signal to the first coupler is operative to allow only signals of a preselected wavelength to be transmitted from the output port of the last coupler to the end user.
In applications where an electronic signal is required for the end use, the optical output signal of the filter may be converted into such a signal by means of a fast photodetector mounted on the output port. In applications where an optical signal is required for the end use, such as for optical spectrum analysis, the output optical data signal may be utilized directly.
In accordance with a further preferred embodiment of the invention, there is provided an optical filter including at least one multiport optical coupler, one connection port of the at least one multiport optical coupler receiving an input optical signal, and another connection port of the at least one multiport optical coupler outputting a filtered optical signal, and at least one tunable optical resonator connected to at least one of the at least one multiport optical coupler.
In accordance with vet another preferred embodiment of the invention, there is provided an optical filter as described above and wherein the at least one multiport optical coupler has at least first, second, third and fourth connection ports, the first connection port receiving an optical signal, the second connection port outputting a filtered optical signal and the at least one tunable optical resonator being connected across the third and fourth connection ports.
In accordance with still another preferred embodiment of the invention, there is provided an optical filter as described above and wherein the at least one multiport optical coupler consists of at least first and second optical couplers, each having at least first, second and third connection ports, the first connection port of the first optical coupler receives an optical signal, the second and third connection ports of the first optical coupler are coupled to the at least one tunable optical resonator, the first and second connection ports of the second optical coupler are coupled to the at least one tunable optical resonator, and the third connection port of the second optical coupler outputs a filtered optical signal.
In accordance with another preferred embodiment of the invention, there is provided an optical filter as described above and wherein the at least one tunable optical resonator is operative to select an optical signal with a specific wavelength or to enable the polarization of the filtered optical output signal to be selected.
In accordance with yet a further preferred embodiment of the invention, there is provided an optical filter with variable finesse consisting of an optical element with variable fin
Majer Daniel
Sadot Dan
Shekel Eyal
Arnall Golden & Gregory LLP
Ben-Gurion University of the Negev Research and Development Auth
Connelly-Cushwa Michelle R.
Lee John D.
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